The present invention relates to a protection apparatus for a control device for use in an electric automobile such as a forklift, called a battery forklift hereinafter.
The battery forklift is provided with a control apparatus for controlling its operation. This control apparatus is provided with a transistor for controlling a power motor and a load motor. The load motor drives an oil pump for executing a loading operation. The transistor is liable to be short circuited due to the influence of temperature or some abnormality. To deal with this fault, a protection device is provided for detecting a short-circuit state in the transistor of the control apparatus and for preventing excess current from flowing through the power motor and the load motor.
In the prior art, a protection circuit is provided for preventing excess current from flowing in the motor, the excess current being caused by a short circuit in the transistor. However, this protection circuit protects against a short circuit fault in the drive transistor of the motor, but not against excess voltage. The battery in a battery forklift must be periodically charged by a separately provided charger electrically connected to it. The charging current is large. The electrical connection between the battery and the charger is made with a plug receiver of the same structure and shape as that between the battery and the control apparatus, and the plug and plug receiver of the same shape are used for the battery, the charger and the control apparatus. This structure is simple and it enables the plug and plug receiver to withstand a large current and to be easily connected with the correct polarity.
When, the plug and plug receiver are of the same shape, as recited above, they are difficult to distinguish from each other because of their similar appearance. Thus, the plug of the charger may be inadvertently connected to the plug receiver of the control apparatus instead of the plug receiver of the battery. If this inadvertent connection occurs, since the output voltage of the charger is naturally higher than the output voltage of the battery which is normally to be connected to the control apparatus, an excess voltage is supplied from the charger to the transistor and motor within the control apparatus. Conventionally, a countermeasure is provided for a short circuit fault of the transistor in the motor drive, but no countermeasure is provided for an application of an excess voltage to a structure part caused by the inadvertent erroneous connection of the plug of the charger to the plug receiver of the control apparatus. Thus, the transistor and the port of the motor would be destroyed by the excess voltage.